1. Field of the Invention
This invention relates to multi-color printing apparatus and, more particularly, to a new, improved arrangement for conveying workpieces between printing stations, and for accurately registering the workpieces at each printing station.
2. Description of the Prior Art
Various methods have been used in the prior art for multi-color screen printing of workpieces including manual and automatic machines. In either of these methods, the workpiece is conveyed to a series of printing stations, each station applying a particular color to the workpiece. Care must be taken to ensure accurate registration of the workpiece at each printing station if the desired multi-colored image is to be attained. The workpiece is carried by a pallet between the various printing stations. Not only must the workpiece be accurately registered initially on the pallet, but also each pallet must itself be accurately registered to each successive printing machine in the printing operation. Significant improvements in turret indexing and registration have been provided by U.S. Pat. No. 4,099,460 issued to Bubley et al. on July 11, 1978 and in a curved Geneva mechanism disclosed in copending patent application Ser. No. 884,540 filed July 11, 1986, now U.S. Pat. No. 4,724,760. The printing arrangement disclosed in that patent includes a central turntable surrounded by a plurality of satellite printing units radially positioned about the turntable. An indexer for rotating the turntable about a central shaft includes a drive disk having equally spaced radially-extending slots for accepting rollers mounted at either end of a rotating drive bar. As the drive arm rotates, a roller enters a slot and continued rotation of the drive arm urges the drive wheel in a circular motion until it emerges from the slot, whereupon the process is repeated with the other roller mounted at the opposite end of the drive bar.
U.S. Pat. No. 4,099,460 also provides registration of each pallet at a printing unit. As cam is provided for rotation with the shaft of the motor which drivingly rotates the drive bar. A roller follower tracks the cam, being raised and lowered thereby at predetermined intervals. The roller is connected to a pivotally mounted arm, causing a free end of the arm to be periodically raised and lowered at a predetermined relationship to the rotation of the drive bar. If the free end of the bar is pinned to a locking member having a forked tip which receives a stub shaft or locking pin cantilevered at one end to the outer edge of the turntable. The lacking member, which is raised and lowered in a generally axial direction, mates with the locking pin upon raising to prevent rotation of the turntable. The forked tip defines a pin-receiving recess closely conforming to the locking pin to ensure accurate registration of the turntable as the pin is received in the recess. The cam is shaped so that, after a printing operation is completed, the roller following the cam is quickly dropped, so as to quickly drop the forked end out of engagement with the locking pin.
While the above-referenced indexing and registration means of U.S. Pat. No. 4,099,460 has proven very satisfactory and has been well-received in the industry, it is now desired to print increasingly larger patterns on workpieces. Not only must larger and more massive pallets supporting the workpieces be moved from printing station to printing station, but the pallets must be located at increasing greater distances from their central drive shaft to allow greater printing strokes in a radial direction. While the forked locking member and lock pin arrangement of U.S. Pat. No. 4,099,460, the curved Geneva mechanism of the aforesaid application have met with great success, larger and more massive turntables are difficult to restrain using this arrangement. The larger mass of the indexer will bang and vibrate if it is a stationary stop before its inertia has been dissipated; also, there may be vibration and bounce. The precise positioning of indexers of ten or twelve foot or more in radius is a difficult problem.